Refining of metals by treatment of molten metal with liquid slag or with molten salt and treatment of molten slag with molten salt are examples of conventional processes where two immiscible molten liquids having different densities are treated.
Conventionally slag treatment of liquid metals is carried out by adding either molten slag or slag forming additives to liquid metal contained in a ladle or the like. The liquid slag and the molten metal are mixed and impurities in the liquid metal are transferred to the slag. After settling the slag is removed from the liquid metal. One important feature with slag treatment is that the slag and the metal must have different densities in order to allow for separation of the slag phase and metal phase.
Treatment of liquid metals with molten salts is carried out in a manner corresponding to slag treatment. Also for treatment of liquid metals with molten salts there must be a difference in the densities of the metal and the salt in order to allow for separation of molten metal phase and molten salt phase.
In some instances liquid slags are treated with molten salts in order to produce a clean slag for later use in slag refining of metals.
In EP-B 699625 it is disclosed a method for removing impurities from molten silicon by slag treatment, particularly to remove boron from molten silicon.
The process according to EP-B 699625 is that slag is continuously or substantially continuously added to molten silicon and that the slag is continuously or substantially continuously inactivated or removed from the silicon melt as soon as equilibrium between the molten slag and molten silicon is reached with respect to the impurity elements to be removed. The process of EP-B 699625 is, however, difficult to carry out in practice, as it is difficult to establish when equilibrium between the molten slag and the molten silicon with respect to the impurities to be removed is reached. Further, inactivation of the slag and full removal of the slag is difficult to carry out in practice.
In EP-B 1441983 it is disclosed an apparatus for continuous slag treatment of molten silicon for removal of one or more impurity elements from the silicon.
The apparatus comprises a vessel for molten silicon and liquid slag having an upwardly extending overflow pipe for slag connected to a slag outlet opening in the bottom of the vessel. An open pipe is arranged about the overflow with is an annulus between the pipe and the overflow. The pipe extends upwards to a level above the top of the overflow and downwards to the bottom of the vessel and the pipe has openings at the bottom of the vessel. A closeable outlet opening for slag treated silicon is arranged in the sidewall of the vessel. In operation a bath of molten silicon is filled into the vessel, and slag is continuously supplied to the top of the silicon bath, sinks through the silicon bath, through the opening in the pipe at the bottom of the vessel and leaves the vessel through the overflow. When sufficient slag has been added, slag and silicon are allowed to rest to allow for separation of silicon and slag whereafter part of the silicon is removed through the closeable opening in the sidewall of the vessel.
Even if the apparatus according to EP-B 1441983 allows for continuous slag addition it is in fact a batch process as molten silicon has to be added and tapped at intervals, and slag addition has to be stopped before and during tapping of slag treated silicon. In addition, the rather complicated design of the apparatus makes it difficult to keep the temperature in the apparatus at the same level causing risk of solidification of silicon or slag in the apparatus.
There is thus need for a true continuous apparatus and method for treatment of two immiscible molten liquids having different densities.